1. Field of the Invention
This invention relates to polymeric materials which are filled with lubricant fillers to reduce the coefficient of friction and increase the wear life of the polymeric material.
2. Description of the Prior Art
It is known that various solid and liquid lubricant materials can be dispersed in a polymeric matrix as by melt extrusion to reduce the coefficient of friction and increase the wear life of the polymeric matrix. It is also known that lubricant fillers can be added to a monomer or a precursor of the polymer followed by subsequent polymerization. For example, U.S. Pat. No. 3,252,905 discloses a composition in which certain low melting lubricant fillers, such as stearone and laurone, are mixed with a lactam monomer which is then polymerized to form a filled polyamide article. Since the melting point of caprolactam and laurone is about 69.degree. C. and the melting point of stearone is only about 88.degree. C., it is convenient to mix these lubricating fillers with the caprolactam while all of the materials are above their melting points.
When an effort was made to duplicate the results shown in the referenced patent for commercial purposes, the initial results indicated that the coefficient of friction was improved as reported in the patent but that a meaningful improvement in wear rate was not obtained. The experiments were repeated using the same materials, but under different mixing conditions, and wear rates comparable to those listed in the patent were then obtained. Microscopic examination of the test specimens revealed that the lubricant filler was in a discrete phase in the samples that showed improved wear life, whereas the lubricant filler had been thoroughly dispersed in the nylon matrix in the unimproved samples. Further investigation confirmed the fact in order to obtain improvements in wear, it was necessary to use nonintensive mixing and preserve the discrete phase of the fillers.
It is also known, as disclosed in U.S. Pat. No. 3,458,596, that when polyethylene lubricant fillers are added to nylon they must be maintained in a discrete phase to obtain substantial improvements in friction and wear. U.S. Pat. No. 3,458,596 teaches that when polyethylene is melt mixed with nylon, ultrahigh molecular weight polyethylene should be used since its melt viscosity is sufficiently high to preserve the polyethylene in a discrete phase. It has not been recognized in the prior art, however, that the principles disclosed in this patent have applicability other than in nylon/polyethylene systems.